The phenomenon existing in two or more crystal forms among the same compound is called as polymorphism, and the individual crystals is called as polymorphs each other. The difference in crystal forms sometimes gives rise to a variation in physicochemical properties such as density, melting point, solubility and stability.
It is therefore useful to study the polymorphism of a candidate pharmaceutical compound and to obtain the compound in a single crystal form that exhibits excellent physicochemical properties. However, it is not easy to find out a single crystal form and a method for supplying such a crystal form. Further, it needs to evaluate the properties of a single crystal found and to conduct intensive study.
Patent Document 1 describes arylalkylamine compounds useful for the prevention or treatment of diseases such as hyperparathyroidism. As an example, there is described a dihydrochloride salt of 4-(3S-(1R-(1-naphthyl)ethylamino)pyrrolidin-1-yl)phenylacetic acid (hereinafter, also written as Compound (A)) represented by the formula below.

However, Patent Document 1 does not describe or suggest the solid state, physicochemical properties or polymorphism of the dihydrochloride salt of Compound (A) (Compound (A′)).
Further, Patent Document 1 describes that the dihydrochloride salt of Compound (A) (Compound (A′)) may be produced as follows:

According to Patent Document 1, Compound (B) is produced as follows:
